So far, attempts to employ a single transistor to block dangerous currents have met with considerable difficulties. When two transistors are placed in series in the primary current path of a circuit to be protected, the voltage drop across one transistor can be used at the gate of the other transistor. However, efforts to use the voltage drop across a single transistor to drive its own gate to provide an efficient and effective device for protecting a circuit during routine, long-term operation have not been successful. When an overcurrent condition arises, that condition must be detected and dealt with quickly. If the single transistor device reacts too slowly, the circuit protected by that device will suffer damage. Moreover, if the single transistor takes too long to enter blocking depletion mode, the single transistor itself will be damaged by the overcurrent passing through that transistor. Similarly, entering a current-limiting mode too slowly could damage the protected circuit and the transistor.
Optionally, an auxiliary power source can be used to control the gate of a single transistor placed in the primary current path of a circuit to be protected from overcurrent conditions. That auxiliary power can be derived from a source other than the circuit to be protected, such as an independent mains supply or a long-life lithium ion battery. However, should the auxiliary power source itself experience an overcurrent condition, or simply disappear as in the case of a discharged battery, the single transistor may be damaged or fail to operate, thereby damaging the circuit to be protected, or leaving that circuit unprotected altogether. Furthermore, the auxiliary power requirement wastes energy, and may cause additional thermal energy dissipation requirements for the protected circuit.
A normally-on transistor may exhibit current-limiting properties in some circumstances, for example, when its gate is shorted to its source, and the voltage drop from its drain to its source exceeds the transistor's characteristic threshold voltage. However, the voltage required to achieve current-limiting behavior is usually unacceptably high for many applications. Moreover, the transistor must exhibit an on resistance of at least several ohms, requiring a significant power loss during normal current conditions. Variability in the threshold voltages of transistors, power loss, heat generated by such a transistor, and other obstacles make it impractical to simply employ a single transistor by itself as a current-limiting device in many circumstances.
Circuit protection devices are needed that do not require an auxiliary power source, more efficiently guard low-power and high-power circuit applications, serve any circuit with strict or sensitive energy requirements, and adequately protect electrical circuits.